Sea Bottom Research Articles

Analysis of the temporal and spatial variability of whale shark (Rhincodon typus) aggregation in the South Ari Marine Protected Area, Maldives, Indian Ocean

Whale sharks are known to aggregate in coastal areas. In the South Ari Marine Protected Area (Maldives) a aggregation, mostly represented by young males with a high level of residency, has been described in the literature. Despite the worldwide interest in the natural resources of the Maldives, this population is increasingly subjected to anthropogenic pressure and major concern regards the flourishing tourist industry. In this study, data collected by the Maldives Whale Shark Research Programme between 2014 and 2017 have been used to detect both temporal and spatial patterns of occurrence. Favourable environmental conditions to visually detect whale sharks have been defined for the studied area. Accordingly, a total of 1077 shark encounters have been analysed in this study. Environmental conditions (i.e. sea surface temperature, monsoon occurrence) have been used to detect possible factors affecting the spatial and temporal variability of Rhincodon typus aggregations. A two-way ANOVA has been performed to detect temporal trends in animal occurrence, sea surface temperature pattern and to investigate the sea bottom depth variability during encounters. Significant differences in the monthly occurrence of whale sharks within the same year and among different years have been detected. Similar patterns have been observed for environmental parameters such as sea surface temperature and depth. A different spatial distribution has also been detected as a function of the Indian Monsoon reversal (north-eastern and south-western) affecting the area. During the northeast monsoon period, whale sharks appeared to concentrate in a smaller longitudinal range closer to the western-central part of the MPA, where deeper water conditions occur due to the proximity of a deep depression (submarine canyon). Results from this study provide new pieces of information for the implementation of dedicated management actions to protect the whale sharks population inhabiting the South Ari Marine Protected Area.

Adaptive Visual Information Gathering for Autonomous Exploration of Underwater Environments

This work presents the development and field testing of a novel adaptive visual information gathering (AVIG) framework for autonomous exploration of benthic environments using AUVs. The objective is to adapt dynamically the robot exploration using the visual information gathered online . This framework is based on a novel decision-time adaptive replanning (DAR) behavior that works together with a sparse Gaussian process (SGP) for environmental modeling and a Convolutional Neural Network (CNN) for semantic image segmentation. The framework is executed in mission time. The SGP uses semantic data obtained from stereo images to probabilistically model the spatial distribution of certain species of seagrass that colonize the sea bottom forming widespread meadows. The uncertainty of the probabilistic model provides a measure of sampling informativeness to the DAR behavior. The DAR behavior has been designed to execute successive informative paths, without stopping, considering the newest information obtained from the SGP. We solve the information path planning (IPP) problem by means of a novel depth-first (DF) version of the Monte Carlo tree search (MCTS). The DF-MCTS method has been designed to explore the state-space in a depth-first fashion, provide solution paths of a given length in an anytime manner, and reward smooth paths for field realization with non-holonomic robots. The complete framework has been integrated in a ROS environment as a high level layer of the AUV software architecture. A set of simulations and field testing show the effectiveness of the framework to gather data in P. oceanica environments.

СУБМАРИННЫЕ ГИДРОГЕОЛОГИЧЕСКИЕ СТРУКТУРЫ И ПОДЗЕМНЫЕ ВОДЫ ОХОТОМОРСКОГО РЕГИОНА

The results of multi-year regional geological, geophysical, oil, and hydrogeological exploration related to the study of seas and oceans were used to characterize the submarine hydrogeological structures of the Sea of Okhotsk region - hydrogeological massifs, artesian and volcanogenic basins. The hydrogeological stratification of the geological cross-section was performed, according to which five Cenozoic aquifer systems were distinguished in the artesian basins. The chemical composition of submarine groundwater was characterized, whose mineralization decreases with depth from 27–30 g/dm3 to14–20 g/dm3. Gases (mainly methane) and thermal waters forming orebodies of metallic sulfides and non-metallic minerals are discharged into the bottom sea waters along zones of tectonic faults.

HOMOGENEOUS LOW-CRESTED STRUCTURES FOR BEACH PROTECTION IN CORAL REEF AREAS

In many countries, the health of the marine ecosystems and the sun-sand-sea tourism depend on the coral reefs, which have been retreating around the world during the last decades. Homogeneous Low-Crested Structures (HLCS), made of large rocks or pre-cast concrete units, can be placed to mimic the functions of beach protection and eventually serve as a refuge for species. HLCS is a type of multi-purpose green infrastructure which is functionally similar to conventional low-crested structures but have higher porosity and are more easily dismantled for re-use. Contrary to conventional low-crested structures, the functionality of HLCS protecting beaches depends on the selected placement grid; this paper describes physical and numerical placement tests on horizontal bottom used to characterize the layers coefficients of Cubipod HLCS. The Bullet Physic Engine (BPE) numerical model used in the gaming industry, which is based on the rigid body method, is calibrated using the physical placement tests. The layer coefficients of Cubipod HLCS measured in the physical placement tests were similar to those obtained with the BPE numerical model, which could be used to optimize placement grids of HLCS on specific sea bottom conditions. Finally, the influence of the placement grid of Cubipod HLCS on the structure height, crest freeboard and wave transmission is analyzed.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/5bi-jpuJYcQ

Characteristics of Very Low Frequency Sound Propagation in Full Waveguides of Shallow Water.

This work is concerned with the characteristics of very low frequency sound propagation (VLF, ≤100 Hz) in the shallow marine environment. Under these conditions, the classical hypothesis of considering the sea bottom as a fluid environment is no longer appropriate, and the sound propagation characteristics at the sea bottom should be also considered. Hence, based on the finite element method (FEM), and setting the sea bottom as an elastic medium, a proposed model which unifies the sea water and sea bottom is established, and the propagation characteristics in full waveguides of shallow water can be synchronously discussed. Using this model, the effects of the sea bottom topography and the various geoacoustic parameters on VLF sound propagation and its corresponding mechanisms are investigated through numerical examples and acoustic theory. The simulation results demonstrate the adaptability of the proposed model to complex shallow water waveguides and the accuracy of the calculated acoustic field. For the sea bottom topography, the greater the inclination angle of an up-sloping sea bottom, the stronger the leak of acoustic energy to the sea bottom, and the more rapid the attenuation of the acoustic energy in sea water. The effect of a down-sloping sea bottom on acoustic energy is the opposite. Moreover, the greater the pressure wave (P-wave) speed in the sea bottom, the more acoustic energy remains in the water rather than leaking into the bottom; the influence laws of the density and the shear wave (S-wave) speed in the sea bottom are opposite.

Ecological analysis of helminth fauna of wild artiodactyles (Mammalia: artiodactyla) in Karakalpakstan

The purpose of the research is studying current fauna of helminths in wild artiodactyls and analysis of their ecological relationships with populations of representatives of the Artiodactyla in biogeocenoses of Karakalpakstan.Materials and methods. Wild populations of artiodactyls were studied in 2016–2020 in the ecosystems of the Kyzyl Kum, Ustyurt and the lower reaches of the Amu Darya, including the drained bottom of the Aral Sea. Some killed animals (the saiga, Persian gazelle, and Bokharan deer) seized from poachers, as well as those killed by predators and feral dogs in the sanctuaries and reserves of Karakalpakstan were used to collect helminthological material. Wild boars were examined after shooting during the hunting season. A total of 11 saigas, 13 Persian gazelles, 7 Bokharan deer and 16 wild boars, and 213 fecal samples from these animals were examined. The studies were carried out using generally accepted methods of dissection (Skryabin, 1928; Ivashkin et al., 1971) and ovoscopy of animal feces. For a comparative analysis of helminth fauna of the study animals, we used previously collected (1959–1978) parasites from the wild boar, saiga, Persian gazelle and Bokharan deer in Karakalpakstan.Results and discussion. We have found that the fauna of the Artiodactyla is represented by the following 5 species in the biogeocenosis of Karakalpakstan: Sus scrofa nigripes Blan., Cervis elaphus bactrianus Lydek., Gazella subgutturosa Gueld., Saiga tatarica Linn., and Ovis orientalis arcal Gmel., in which we identified 26 helminth species classified as Cestoda, Trematoda and Nematoda. Cestodes are represented by 6 species, trematodes by 2 species, and nematodes by 18 species. The helminth species composition of the study animals is 14 species in pigs, 11 species in Bokharan deer, 13 species in saigas and 14 species in Persian gazelles. Representatives of the genera Taenia, Echinococcus, Fasciola, Schistosoma, Gongylonema and Setaria are common to these animal species. By nature of the biological cycle, the helminths of the study artiodactyls recorded by us can be divided into two groups: monoxenous and heteroxenous. Monoxenous parasites include representatives of the Trichocephalidae (3 species), Trichostrongylidae (2 species), Dictyocaulidae (2 species), Syphaciidae (1 species), and Ascarididae (1 species), whose life cycles proceed without changing hosts. Eighteen species of representatives of cestodes (6 species), trematodes (2 species) and nematodes (10 species) are developed with the involvement of intermediate hosts.

Mineralogy and geochemistry of deeply-buried marine sediments of the Vaca Muerta-Quintuco system in the Neuquén Basin (Chacay Melehue section), Argentina: Paleoclimatic and paleoenvironmental implications for the global Tithonian-Valanginian reconstructions

The Vaca Muerta-Quintuco (VM-Q) system of Tithonian-early Valanginian age was studied in the Chacay Melehue section of the Neuquén Basin (western Argentina) by means of sedimentological, mineralogical and geochemical analyses in order to determine the main driving factors that triggered the paleoenvironmental change from a carbonate ramp (Vaca Muerta Formation, VMFm) to a mixed siliciclastic/carbonatic marine environment (Quintuco Formation, QFm). The VMFm was divided into two stratigraphic intervals: Lower VMFm (LVMFm) and Upper VMFm (UVMFm), whereas the QFm is subdivided into the Puesto Barros Member (PBMb) and the Cerro La Visera Member (CVMb), which can be correlated to other sections in the basin (e.g., Puerta Curaco). Isolated, turbiditic sandstone beds, correlated to the Huncal Member, are included in the QFm.The LVMFm (Tithonian) and the UVMFm (Berriasian) are constituted by dark, well-laminated marls, mudstones, calcite concretions and tuffs. The PBMb (earlier early Valanginian) is constituted by marls and sandstones, whereas the CVMb (later early Valanginian) is constituted by marls, mudstones, siltstones, sandstones and coquinas. The LVMFm (Total organic carbon, TOC ~ 1–4 wt%) is characterized by the enrichment of redox sensitive trace elements (RSTE), where the enrichment of Ni and Cu suggest high productivity in the water column, and the enrichment of Mo, U, V points to sea bottom anoxia, with periods of increased oxygenation as deduced from higher P concentrations in marls and mudstones. The clay mineral association is constituted by mixed-layer illite/smectite formed by the transformation of smectitic layers. The predominance of smectite in coeval sucessions, less overprinted by burial diagenesis, suggests a temperate and semi-arid climate in the adjacent continent. The UVMFm (TOC ~ >1 wt%) is characterized by a gradual decrease of the RSTE pointing to a decrease in productivity and a slight increase in the oxygenation of the sea bottom. In addition, a change towards more humid conditions in the continent is inferred by both the increase in the Chemical Index of Alteration and the Al2O3/TiO2 ratio. The PBMb (TOC ~1 wt%) has even lower content of RSTE indicating diminished sea water productivity and a gradual rise of the oxygenation of the sea bottom. In this interval, increased illite contents suggest periods of enhanced physical weathering, probably related to the tectonic uplift of the Huincul Ridge. The RSTE in the CVMb (TOC <1 wt%) documents a fully oxygenated sea bottom, where the productivity of the water column was negligible. The presence of kaolinite and the increment of the detrital sedimentation in the CVMb indicate a change towards more humid conditions in the hinterlands. The enhanced runoff caused by this paleoclimatic change towards more humid conditions that started in the early Berriasian and increased during the later early Valanginian triggered the change from carbonate ramp to mixed siliciclastic/carbonatic to siliciclastic marine paleoenvironments. The organic carbon isotope composition (δ13Corg vs. VPDB) of the VMFm ranges between −30.0 and −23.4‰, whereas in the QFm values range between −29.0 and −23.9‰. Two positive carbon isotope excursions (PCIE) are recorded in the system: PCIE-A in the lower part of the VMFm (early Tithonian) with δ13Corg values ~ −25‰, and PCIE-B in the upper part of the QFm (later early Valanginian) with δ13Corg values ~ −24.5‰. The shift in δ13Corg at PCIE-B is up to +4.2‰ and marks the onset of the Weissert Event. This is the first time that the onset of this event is recorded in the Neuquén Basin, within the Lissonia riveroi ammonite zone. The results of our study confirm that clay mineralogy, trace elements and stable isotopes are valuable proxies of past ocean-climate variability even in sediment deposits that underwent near-surface and deep burial diagenesis.

Bigger juveniles and smaller adults: Changes in fish size correlate with warming seas

Abstract Increasing sea temperatures are predicted to decrease body size of marine ectotherms based on the temperature size rule. This will impact fisheries yields, but empirical evidence of the process is still limited. We used fishery‐independent bottom trawl survey data from 1970 to 2017 to examine the trends of length‐at‐age of four commercially important demersal fish species (cod, haddock, whiting and saithe) in two study areas facing increasing sea temperatures: the West of Scotland and the North Sea. We then compared the trends of length‐at‐age with annual bottom sea temperatures. The mean length‐at‐age of adults declined over the study period, in all species and in both areas, except for cod in the West of Scotland. A common trend of decline in adult length was inversely correlated with bottom sea temperatures. Correlations with temperature at seven yearly time‐lags were significant and negative in the North Sea. Correlations were only significant at lags of 1 and 2 years in the west of Scotland, where sea temperature warming was twice as slow. The mean length‐at‐age of juveniles concurrently increased, which has not hitherto been reported. This trend, shared by all species and both regions, correlated positively with rising temperature, suggesting that our study species have a faster growth rate due to increased temperatures. Synthesis and applications. We examined the body size of a range of commercially exploited fish species, at different age groups, from two management regions. We found that juvenile fish have been getting bigger and adults smaller in both regions. These changes were correlated with rising sea temperatures, providing empirical evidence that global warming is affecting the size of commercial fish species. The effects of these changes on productivity of fish populations and fisheries yield now require investigation. Temperature changes should, therefore, be included into forecasts used in fisheries science in order to mitigate the impact of global warming and maximise sustainable yields.

Using Remote Sensing Techniques to Document and Identify the Largest Underwater Object of the Baltic Sea: Case Study of the Only German Aircraft Carrier, Graf Zeppelin

New shipwrecks are usually found during other work at sea. In 2006, the research vessel St. Barbara discovered probably the most massive wreck that lies on the bottom of the Baltic Sea. Its identity was unknown, and the Arctowski was sent to conduct research. In this study, the author describes how an adequately selected research methodology using remote sensing technology can lead to obtaining precious and accurate measurement data. The article proposes an appropriate approach to identifying the most massive underwater object in the Baltic Sea. The results of bathymetric and sonar tests are presented. Unconventional use of scanning sonar, which was undoubtedly not foreseen by the manufacturer, was also proposed. The article also shows how some remote sensing techniques support and complement each other when verifying objects on the bottom. Moreover, this paper addresses the impact of sonar frequency and survey methodology on image resolution and data quality. If imaged under the right conditions, side-scan sonar can deliver records based on which shipwreck identification is possible. The wreck of the aircraft carrier lies at the bottom of the Baltic Sea at a depth of about 90 m. Some difficulties in measurements were caused by the lack of a depressor, the use of a light cable line, or the lack of a standard side-scan sonar winch. However, these limitations did not significantly affect the quality of the recorded data. The research results prove that the proposed methodology for using side-scan sonar (SSS) or scanning sonar is correct, and the obtained measurement data make it possible to identify the detected object at the level of 99%.

Spatial Variability of Antarctic Bottom Water in the Australian Antarctic Basin From 2018–2020 Captured by Deep Argo

AbstractThere are two varieties of Antarctic Bottom Water present in the Australian Antarctic Basin (AAB): locally produced Adélie Land Bottom Water (ALBW) and distantly produced Ross Sea Bottom Water (RSBW). Between 2014 and 2018, RSBW has rebounded from a multidecade freshening trend. The return of the salty RSBW to the AAB is revealed by six Deep Argo floats that have occupied the region from January of 2018 to March of 2020. The floats depict a zonal variation in temperature and salinity in the bottom waters of the AAB, driven by the inflow of RSBW. A simple Optimum Multiparameter Analysis based on potential temperature and salinity gives a sense of scale to the composition of the bottom waters, which are nearly 80% of the new, salty RSBW in the south‐east corner of the basin by 2019 and generally less than 40% to the west closer to the ALBW outflow region and the abyssal plain.

Movement tracing and analysis of benthic sting ray (Dasyatis akajei) and electric ray (Narke japonica) toward seabed exploration

Creation of a seabed map is a significant task for various activities including safe navigation of vessels, commercial fishing and securing sea-mined resources. Conventionally, search machines including autonomous underwater vehicles or sonar systems have been used for this purpose. Here, we propose a completely different approach to improve the seabed map by using benthic (sting and electric) rays as agents which may explore the seabed by their autonomous behavior without precise control and possibly add extra information such as biota. For the first step to realize this concept, the detail behavior of the benthic rays must be analyzed. In this study, we used a system with a large water tank (10 m × 5 m × 6 m height) to measure the movement patterns of the benthic rays. We confirmed that it was feasible to optically trace the 2D and 3D movement of a sting and an electric ray and that the speed of the rays indicated whether they were skimming slowly over the bottom surface or swimming. Then, we investigated feasibility for measuring the sea bottom features using two electric rays equipped with small pingers (acoustic transmitters) and receivers on a boat. We confirmed tracing of the movements of the rays over the sea bottom for more than 90 min at 1 s time resolution. Since we can know whether rays are skimming slowly over the bottom surface or swimming in water from the speed, this would be applicable to mapping the sea bottom depth. This is the first step to investigate the feasibility of mapping the seabed using a benthic creature.

Specific Features of the Design and Operation of Magnetic Resonance Mass Spectrometers

Magnetic resonance mass spectrometers (MRMSs), developed in the second half of the 20th century at the Ioffe Institute, have still possessed such a complex of parameters that they have no alternative for solving many analytical problems, in particular, in the isotopy of helium. With the resolution of one of the devices several thousand at the base of the 3He+ mass peak, its absolute sensitivity is <106 atoms of this isotope in the volume of the mass analyzer chamber (~2.5 L), and the 3He/4He isotope ratio measured in a single experiment can be as low as 109 and even 1010. Another device of this type demonstrated resolution of more than 150 000 at the base of the mass peak, which ensured the detection of a 3He+–3T+ doublet. Excellent analytical characteristics of the MRMSs make it possible to use them to study the isotopy of helium in natural and man-made samples (solid, liquid, or gaseous) in meteorites and lunar soil, ferromanganese nodules from the bottom of seas and oceans, in ice cores from the well of Lake Vostok in Antarctica, in water from mountain streams of Mongolia and geysers of Iceland, in volcanic gases of Kamchatka and gases released from structural materials of a modern thermonuclear installation. In this work, we consider the principle and modes of operation of an MRMS and its design. The focus is on the details that distinguish these instruments from other instruments and ensure extremely high analytical performance, which is partly presented in the article.

Sedimentological Characterisation Of Sea Bottom Samples Collected Offshore Muara And Tutong, Brunei Darussalam

The study aims to get some additional knowledge on the modern seafloor composition offshore Brunei Darussalam by looking at the recent stratigraphic succession of the deposited sediments and their distribution patterns. For this reason, 10 shallow cores (22 to 46 cm thick) have been collected by scuba diving along two depth transects spanning from water depth of 20 to 60 m. One of the transects has been sampled north-northwest of the Muara village, just in front of the Brunei Bay and the other one off the coast near Tutong town, away from major sedimentary inputs. The results obtained portray two different sea bottom compositions and two different depth-related sediment distributions. The Muara transect is highly rich in mud and yielded abundant biogenic component at all investigated depths. The Tutong transect has a higher sand content but display constant changes along with depth. The sediment is mostly composed by biogenic grains such as rests of sponges, foraminifera, molluscs and echinoderms; the not biogenic grains are for the vast majority made of quartz. The sandy fractions of both transects have been tested for cyclicity and all cores can be described by functions with comparable periods, thus indicating that an oscillatory environmental event such as the alternation of the monsoon seasons, has similar influence on the seafloor of both transects.

Sound Field Modeling with Bottom Reflective Parameters (P,Q) Based on Parabolic Equation

Acoustical properties of the sea bottom can be described using geoacoustic (GA) models. Most existing propagation models use GA parameters as the bottom properties. It is difficult to obtain GA parameters for a layered bottom because of inter parameter coupling. These problems can be solved by inverting the model-independent reflective parameters P and Q. For a multilayered bottom, a sound field computation model, RamPQ, is developed using the mapping of GA and (P, Q) spaces. The mean square error of the transmission loss in numerical simulations and experimental data for low-frequency sound propagation are employed to validate RamPQ and demonstrate the performance of the model.

UAV-Derived Multispectral Bathymetry

Bathymetry is considered an important component in marine applications as several coastal erosion monitoring and engineering projects are carried out in this field. It is traditionally acquired via shipboard echo sounding, but nowadays, multispectral satellite imagery is also commonly applied using different remote sensing-based algorithms. Satellite-Derived Bathymetry (SDB) relates the surface reflectance of shallow coastal waters to the depth of the water column. The present study shows the results of the application of Stumpf and Lyzenga algorithms to derive the bathymetry for a small area using an Unmanned Aerial Vehicle (UAV), also known as a drone, equipped with a multispectral camera acquiring images in the same WorldView-2 satellite sensor spectral bands. A hydrographic Multibeam Echosounder survey was performed in the same period in order to validate the method’s results and accuracy. The study area was approximately 0.5 km2 and located in Tuscany (Italy). Because of the high percentage of water in the images, a new methodology was also implemented for producing a georeferenced orthophoto mosaic. UAV multispectral images were processed to retrieve bathymetric data for testing different band combinations and evaluating the accuracy as a function of the density and quantity of sea bottom control points. Our results indicate that UAV-Derived Bathymetry (UDB) permits an accuracy of about 20 cm to be obtained in bathymetric mapping in shallow waters, minimizing operative expenses and giving the possibility to program a coastal monitoring surveying activity. The full sea bottom coverage obtained using this methodology permits detailed Digital Elevation Models (DEMs) comparable to a Multibeam Echosounder survey, and can also be applied in very shallow waters, where the traditional hydrographic approach requires hard fieldwork and presents operational limits.

Challenge of monitoring cohesive movement in homing fish using fine-scale 3D positioning

Rockfish, which are well known for their site fidelity and homing ability, live sympatrically with many conspecifics. Conspecifics may be external drivers influencing rockfish movement, and rockfish may move cohesively while travelling. We tested whether rockfish formed a group when returning to their original habitat after artificial displacement and examined the routes they travelled to return home. A fine-scale multi-individual simultaneous positioning method was used to observe the movement trajectories of tagged fish. Our results showed that tagged fish, released in groups, returned to their original habitat (5 of 8 fish) but generally did not travel with other individuals. There was one exception in which 2 individuals moved together for ~100 s immediately after release. These 2 fish had no designated leader, alternating as leader and follower. Our hypothesis was partially corroborated by these rockfish possibly travelling cohesively. The returning fish tended to travel along the sea bottom and the coastline, independent of current; thus, they likely used visual cues, rather than olfactory or social cues, to return home.

Effects of swells on sound propagation in surface duct environment in shallow water

Surface duct is a common duct due to strong sea winds and sea-atmosphere interactions in winter and it is an excellent waveguide in which energy may propagate a long distance. However, the rough interface formed by sea surface waves will seriously damage this excellent performance. In this study, the experimental data of sound propagation over the continental slope in the South China Sea are used to analyze the characteristics of sound propagation in a surface duct. Modeling analyses based on the parabolic equation model RAM and ray trace theory BELLHOP are used to examine these characteristics. The parameters of sea bottom, source depth, wind-driven sea surface, and swell-containing sea surface are taken into consideration in the model. The results show that when the source is located in the surface duct, the parameters of the sea bottom have little influence on sound propagation, while the change of source depth exerts some effects on the sound propagation. By combining the Pierson Moscowitz (PM) spectrum with Monte Carlo method, the rough sea surface is investigated. Since the PM spectrum is related only to wind speed, the wind-driven sea surface is generated by using the actual wind speed measured by the shipborne anemometer. The swell-containing sea surface is defined as a superposition of a sinusoidal pressure-release surface and the wind-driven sea surface. By comparing the effects of two sea surfaces on sound propagation, it is found that when the wind speed is small, swells play an important role in the surface-duct propagation. Experimental data show that for the acoustic signal with a center frequency of &lt;inline-formula&gt;&lt;tex-math id="M3"&gt;\begin{document}$1000\;{\rm{Hz}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M3.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M3.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, the swell-containing sea surface brings around &lt;inline-formula&gt;&lt;tex-math id="M4"&gt;\begin{document}$10 \;{\rm{dB}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M4.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M4.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; loss to a distance of &lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$70 \;{\rm{km}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M5.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;. For the two kinds of rough sea surfaces, rays at launch angles of &lt;inline-formula&gt;&lt;tex-math id="M6"&gt;\begin{document}$\pm 1^{\circ}, 0^{\circ}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M6.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M6.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; are plotted to examine their effects on sound propagation. The results indicate that the swell-containing sea surface which has greater roughness makes rays go toward the sea bottom, thus resulting in larger loss. Therefore, in order to investigate the characteristics of the sound field in the northern South China Sea in winter, especially with high frequency sound signals, the influences of not only winds and waves, but also the swells from the surrounding sea should be taken into consideration. It is important to study the characteristics of sound propagation with swells for improving the performance of sonar equipment in poor sea conditions.

OLE: A Novel Oceanic Lidar Emulator

Oceanic lidar is an effective tool for detecting water’s vertical structure. Cost-effective design of an oceanic lidar system and processing algorithms requires an effective lidar simulator. In this study, an oceanic lidar simulation tool was developed, which is available to the public ( <uri xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">https://github.com/soedchen/OLE</uri> ), and named the Oceanic Lidar Emulator (OLE). OLE is an improved semianalytic Monte Carlo-based lidar simulator, which has the capability of dealing with the physics of light propagating through wind-driven rough air–water interface into the ocean, being scattered by subsurface phytoplankton and reflected from the sea bottom, and returning to the lidar receiver. OLE has three main features: 1) it can deal with stratified water, while most existing lidar models can only be used for homogeneous water; 2) it can be applied to arbitrary scattering phase function (SPF) (e.g., Fournier–Forand or Petzold), while most existing lidar models can only use the Henyey–Greenstein SPF due to the difficulty of solving the inverse equation for the cumulative distribution function; and 3) it takes general consideration of lidar system observation geometry (e.g., receiver field of view, receiving aperture, altitude, incident angle, and so on) and environmental parameters (e.g., wind-driven rough water surface, vertical structure of water optical properties, and sea bottom albedo). We studied lidar extinction caused by multiple scattering and the effects of the SPF, rough sea surface, and stratified water, by comparing simulation results with measurements. These results demonstrated that our model is effective for oceanic lidar simulation.

Damage or benefit? How future scenarios of climate change may affect the distribution of small pelagic fishes in the coastal seas of the Americas

Species occurrence and distribution have already been directly affected by climate change, a scenario that is likely to be accentuated as the temperature rise is expected to exceed 2 °C by 2100. Owing to climate change, organisms are forced to migrate or adapt to new climatic conditions, and if they fail to do so, they are at risk of declining and becoming extinct. However, some species are adapted to overcome and even benefit from these new conditions, increasing their occurrence area or even their abundance. Using Bayesian Species Distribution Models (B-SDMs), we evaluated the current distribution of two halfbeak fishes, Hyporhamphus unifasciatus and Hemiramphus brasiliensis, and the effect of climate changes predicted for 2050 and 2100 on the distribution of these populations in coastal waters of the Americas. We used species occurrence data from bibliographical sources and online databases. One biotic (net primary production - NPP) and four abiotic variables (sea surface temperature - SST), sea surface salinity - SSS), depth, and sea bottom rugosity) were used as potential predictors of species distribution. Results indicated that both species are more likely found in shallower, warmer, and saltier waters. Model prediction suggests that they will probably benefit from climate change, with potential increase in their occurrence area in coastal regions of the Americas, especially due to temperature rise and increased salinity.

Direct Numerical Simulations of the Pulsating Flow over a Plane Wall

The results of direct numerical simulations of the flow generated in a plane duct by a pressure gradient which is the sum of two terms are described. The first term of the pressure gradient is constant in space but it oscillates in time whereas the second term is constant both in space and in time. Therefore, a pulsating flow is generated, similar to that generated at the bottom of a monochromatic propagating surface wave when nonlinear effects are taken into account. The simulations are carried out for values of the parameters similar to those considered in previous investigations. It is shown that even a small constant pressure gradient influences the flow regime in the bottom boundary layer. In particular, turbulence strength is damped when the steady velocity component has the direction opposite to the oscillating velocity component whereas turbulence strength increases when the steady and oscillating components point in the same direction. Even though the flow is not exactly equal to that generated at the bottom of sea waves, where second order effects in the wave steepness induce a steady streaming in the direction of wave propagation, our results provide information on the interaction of the steady streaming with the oscillatory flow and are also relevant for investigating the dynamics of sediment close to the sea bottom. Indeed, since the turbulent eddies tend to pick-up the sediment from the bottom, it can be inferred that the triggering of turbulence enhances sediment transport towards the shore.